I've Been Spotting In Between Periods For A Few Weeks

I've been spotting in between periods for a few weeks now and started having pelvic pain approx 2 weeks ago. I'm on BC pills and just started my regular menstrual cycle. I was referred to have a pelvic ultrasound due to the pain and the only thing found was "free fluid adjacent to the uterus" other than that it was a normal exam. I don't have any other symptoms and I've only been with one partner, I also don't have history of any std's. What could be the cause?